Band-tail states meditated visible-light-driven overall water splitting in Y2Ti2O5S2 photocatalyst

Abstract

Oxysulfide photocatalyst Y₂Ti₂O₅S₂ is a narrow bandgap semiconductor that achieves overall water splitting via one-step photoexcitation under a wide range of solar spectrum (<640 nm). However, the photophysical properties that enable the visible-light-driven overall water splitting in Y₂Ti₂O₅S₂ are not fully understood. Here, temperature/power-dependent and time-resolved photoluminescence spectroscopies reveal the transition of luminescence mechanism from exciton and free carrier recombination at low temperature to band-tail recombination at room temperature. Importantly, the band-tail states help to sustain a long carrier lifetime in Y₂Ti₂O₅S₂, which is beneficial to achieving overall water splitting. Meanwhile, a high density of band-tail states may negatively affect photocatalytic activity due to the trap of photocarriers. Density function theory calculations reveal possible origins of the band-tail states, i.e., spatial potential fluctuations resulting from the random distribution of O_S and S_O antisites. Our findings reveal a new carrier management mechanism for photocatalysis that could guide the design of more efficient Y₂Ti₂O₅S₂ photocatalyst.